The present invention relates to an ion beam apparatus adapted for us in an ion microprobe analyzer or in an ion implantor and, more particularly, to an improvement in an ion source for an ion beam apparatus.
In conventional secondary ion mass spectrometry, as disclosed in U.S. Pat. No. 3,840,743, gaseous component elements such as O.sub.2.sup.+, Ar.sup.+, Ne.sup.+, or the like, are generally used as primary ions.
FIG. 1 shows the principle of operation of a typical conventional secondary ion mass spectrometer. The secondary ion mass spectrometer consists mainly of a primary ion beam illumination system and a scanning ion microscope. The primary ion beam illumination system is intended for generation of an ion beam and for controlling the beam so that an ion beam of desired intensity and size may be applied to the desired portion of the specimen surface.
Usually, the primary ion beam illumination system consists of a gas-leakage controlling device 1, cathode 2, intermediate electrode 3, anode 4, extracting electrode 5, condenser lens 6, object lens aperture 7, objective lens 8, electrostatic deflector 9, power source 19 for the filament, arc discharge power supply 20, accelerating power supply 21, and a lens power supply 22.
Ion beam 10, emitted from an ion gun (constituted by members 1, 2, 3, 4, and 5), is focused on a specimen 11 by a lens system 6, 8. The primary ion beam can be applied to any desired portion on the specimen, or can even scan over the area of the specimen as in the case of television, by a suitable control of the electrostatic deflector 9 and a scanning generator 28. Numeral 23 denotes a secondary ion acceleration power supply.
The mass spectrometer is constituted by a secondary ion extracting electrode 12, electrostatic sector 13, .beta. slit 15, secondary ion detector 16, electrostatic power supply 24, magnetic sector field 14, and its power supply 25, amplifier 26 and a secondary ion pick-up device 27. In operation, the secondary ions 18 generated by primary ion bombardment, are separated according to their mass to charge ratio by the magnetic sector field 14, are detected by a detector, and are then read out by the pick-up device 27. These ions are utilized for brightness modulation of CRT 17, as necessary.
A scanning type ion microscope consists of an auxiliary apparatus such as the aforementioned primary ion beam illumination system and mass spectrometer, and a CRT.
The arrangement is such that the primary ion is caused to scan in synchronization with the electron beam of the CRT, and the secondary ions emitted from the specimen are separated in accordance with mass to charge ratio and picked up as a specific ion which is to be used as a brightness modulation signal for CRT, so as to provide the elemental map of the specimen surface.
Usually, in the apparatus as shown in FIG. 1, a duo-plasmatron type ion source is used as the ion gun, so that the primary ions are produced by an electric discharge. This means that the element to be picked up in the form of ions has to be in gaseous phase. Therefore, such an apparatus can be applied only to limited use.
Also, the ionization coefficient of the element under application of ions largely depends on the kind of elements. FIG. 2 shows ionization coefficients of various elements bombarded by electronegative (O.sup.-) ions, where O.sup.- or Ar.sup.+ ions are used, the ionization coefficient is extremely high for the elements such as Be, Mg, Al, Ca, In, and Be, but is quite low for elements such as S, As, Se, Cd, Te, Au, and Pt. Therefore, the sensitivity of the apparatus is much smaller for the analysis of elements such as As, Cd, Se, Te, or Au, than for these elements such as Be, Mg, or Al. This presents one of the problems inherent in the secondary ion mass spectrometer.
Meanwhile, also in the field of semiconductors, ions such as of B, As, Te, and the like, are used in a technique called ion injection. In this case, these ions are produced by ionizing a gaseous compound containing these elements, also by means of an electric discharge. Therefore, it is quite indispensable that the compound exists in the gaseous phase and, therefore, it is quite difficult to obtain solely the ion of the desired element.
The specification of U.S. Pat. No. 3,631,283 has been known as disclosing a method which makes use of a solid ion source.
According to this description, an evaporation source is provided in the ion source. The solid material is heated and evaporated into gaseous phase for ionization. This method, however, suffers from practical problems that only these elements or compounds having a low melting point and low vapour pressure can be treated by this method, that the ion beam is rendered unstable due to unstable evaporation, that the ion source is prone to be overheated to cause melting down of electrodes, and other members, and that the ion source is seriously contaminated.
A surface ionization type ion source for cesium, which functions in a manner substantially the same as that of the aforementioned method making use of solid resource, is disclosed in a document for conference of study on electronic devices (Document No. EDD-74-22, Feb. 26, 1974) published from Electric Society of Japan. Briefly, this apparatus has the following construction. Two tungsten heaters are disposed in a cylinder (inner diameter 16 mm, length 25 mm) made of nickel. Between these heaters, a mesh of tantalum is stretched. The portion closer to the ion extracting aperture is adapted to be heated to a high temperature by one heater, so as to ionize the cesium atom through the surface ionization, while the portion separated from the first mentioned portion by the tantalum mesh functions as a furnace for evaporating the cesium.
The temperatures of these portions can be controlled independently of each other, by respective heaters.
Namely, the ionization portion is heated to a temperature which is not so high but sufficient to avoid the evaporation deposition of cesium, while the furnace portion is heated to provide an optimum evaporation rate of cesium, and need not be heated to a high temperature.
The cesium ion extracted from the ionization portion through the aperture of 1 mm diameter is then focused by a lens and reaches the specimen surface.
The ion current which reaches the specimen surface depends mainly on the evaporation rate of cesium and, therefore, rapidly grows as the power supply to the heater of ion source is increased.
Although the increased ion current shortens the life, an ion current as large as several micron amperes can readily be obtained by this apparatus. As a loading material or filler, used is cesium carbonate, cesium sulfate, cesium chromate or the like.
This apparatus also relies upon heating for obtaining ion and, accordingly, suffers from the same problems as those pointed out in relation with the aforementioned apparatus.
FIG. 3 shows the relationship between the relative ionization coefficient and the atomic number of secondary ions, when Cs.sup.+ ions produced by this apparatus is used as the primary ion. From the comparison of FIGS. 2 and 3 with each other, it will be seen that S, As, Se, Cd, Te, and Au which all exhibit quite small ionization coefficients in FIG. 2, show extremely large ionization coefficients in FIG. 3 due to the bombardment by Cs.sup.+ ions.
Thus, the same element exhibits different ionization coefficients by the use of a different primary ion. This leads to the conclusion that the secondary ion analysis of high accuracy can be obtained by suitable different primary ions.
Recently, a proposal to improve the heat-resistant property and surface hardness of metal has been carried out by injecting ions into the metal surface. This also serves to increase the demand for a solid ion source. However, a solid ion source which can overcome the aforementioned problems sufficiently well has not been available up to now, so that the progress of this field of industry has been hindered.